Edwina Dorsey Valves April 14th, 2019 - 12:33:25
Thus, it is also very necessary to learn something about the dead band. It is a major contributor to excess process variability, and control valve assemblies can be a primary source of dead band in an instrumentation loop due to a variety of causes such as friction, backlash, shaft windup, relay or spool valve dead zone, etc. It is a general phenomenon where a range or band of controller output values fails to produce a change in the measured process variable when the input signal reverses direction.
Ball valves are usually quarter turn valves (as we turn the lever by 90 degrees to start or stop the flow) and use a hollow, perforated and pivoting ball that is also called as a floating ball. It is through this valve that the flow of fluids takes place. When the lever or handle is turned to 90 degrees, the hole of the ball gets aligned with the pipe opening and starts the flow. When again the lever is turned by 90 degrees, the hole turns around and the ball blocks the opening of the pipe thus causing the flow to stop.
The second type is a product with override control to stop flow regardless of flow direction or pressure. It can not only close in response to backflow or insufficient forward pressure, but also can be deliberately shut by an external mechanism, thereby preventing any flow regardless of forward pressure.
There is a slight flaw with valves though. The valve combination of 2-3 will be slightly sharp, the 1-3 combination will always be quite sharp, and the 1-2-3 combination will always be very, very sharp. Lets explore why this phenomenon happens.
Among the many types of Valves manufactured by Swagelok, are Ball Valves - these are used where flow needs to be completely shut-off. They have advantages over many other types of Valves in that they are easy to use, easy to maintain, and can regulate high-pressure, high-volume and high flow of temperature. They are uncomplicated, sturdy, and inexpensive; they lend themselves to a long service life. Swagelok has a full catalogue of various types of Ball Valves. For all the advantages of Ball Valves, they are not suitable for use where fine control of a Valve is needed. Depending on system requirements, in these circumstances, a Needle Valve may be appropriate - these allow for a range of flow, and can control flow into delicate gauges where sudden surges of pressurized fluid can cause damage. They can also be used where flow must be precisely and gradually brought to a halt. Needle Valves are not suitable for simple shut-off applications as are Ball Valves - they should be used where a small rate of flow is required.
So valved instruments are set up so that each valve, individually is in tune. Problems occur when performers must use valve combinations to adjust the pitch by more than three half steps. As you can see from the previous calculations, each time you add another half step, the working length must increase by more than the previous increase. Using the example of a 100" instrument, the third valve increases the length to 121.36" to produce an in-tune note three half steps below the original pitch. To lower the pitch a half step past this note, 8.09" of tubing is required. However, because the 2nd valve's length is only 6.67" this combination will be slightly sharp. This problem only compounds itself and in the 1-3 and 1-2-3 combinations, the deficit between the actual length and the "in-tune" length is 2.94" and 5.04" respectively. As you can tell, this creates a big problem, in fact, the 1-2-3 combination is about a fourth-step sharp!